Twilight, that time of day when the sun is not directly visible because it is below the horizon, has inspired poetry, paintings, stories, and music as far back as we can trace in history. It seems to induce a mystical state in all of us when we find the time to stand and observe it’s eternal wonder.
The Yaqui Indians, who practiced the Toltec system of knowledge in the southwestern U.S. and northwestern Mexico, say that Twilight is the “crack” between the worlds, or between this dimension and the next higher one. That meant that if you had the knowledge, you could enter a higher dimension during that time of day.
Scientifically, twilight is defined by the solar elevation angle, which is the position of the geometric center of the sun relative to the horizon.
When the sun is between 0° and 6° below the horizon. The brightest stars are visible during this time, such as Venus, the “morning star” or “evening star.”
When the sun is between 6° and 12° below the horizon. During this time, sailors can take visible star sightings of the known stars, and still use the horizon as a point of reference.
Defined by the sun being 18° or more below the horizon. This is the time when the sky is considered dark enough for astronomical observation. To the common person, this is the time of night they define as dark. For the astronomer, only the amount of light or air pollution would determine what is visible in the sky during astronomical twilight.
At latitudes greater than 48.5 degrees North or South, twilight can last from sunrise to sunset near the summer solstice, because the sun never goes more than 18° below the horizon. That in turn means the sky never becomes totally dark.
Cities in northern latitudes, such as Helsinki, Oslo, Stockholm, Tallinn, and St. Petersburg, do not receive civil twilight from sunrise to sunset, but they have noticeably lighter skies at night known as “white nights” around the summer solstice.
On the next pleasant evening, go out for a walk, and see if you can find the ‘crack’ between the worlds.